The only thing that can be done now is to contact eBay directly and request a FVF credit - often if you have a clear indication in messages that the buyer didn't want to proceed with the transaction, eBay will credit the fees even if the buyer refuses the cancellation.

Generally, depending on circumstances, I advise buyers that I'm sending through a cancellation request beforehand, explaining that it finalises the transaction and releases them from the obligation to pay. In this case I probably would have replied to their message first. It's also a good way to test how responsive and/or receptive they are.